Process fluid or gas bearings are utilized in a number of diverse applications. These fluid bearings generally comprise two relatively movable elements with a predetermined spacing therebetween filled with a fluid such as air, which, under dynamic conditions, form a supporting wedge sufficient to prevent contact between the two relatively movable elements.
Improved fluid bearings, particularly gas bearings of the hydrodynamic type, have been developed by providing foils in the space between the relatively movable bearing elements. In a typical journal bearing, the relatively movable bearing elements comprise a stationary housing having a bore, and a rotating member such as a shaft with a bearing surface passing through the bore. The foils are mounted to the housing within the bore in a surrounding relationship to the bearing surface of the rotating member. Such foils, which are generally thin sheets of a compliant material, are deflected by the hydrodynamic film forces between adjacent bearing surfaces and the foils thus enhance the hydrodynamic characteristics of the fluid bearings and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, these foils provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and dampening effect. The ready availability of relatively clean process fluid or ambient atmosphere as the bearing fluid makes these hydrodynamic, fluid film lubricated, bearings particularly attractive for high speed rotating machinery.
In order to properly position the compliant foils between the relatively movable bearing, elements, a number of mounting means have been devised. In journal bearings. At it is conventional practice to mount the individual foils in a slot or groove in one of the bearing elements as exemplified in U.S. Pat. No. 3,615,121.
To establish stability of the foils in most of these mounting means, a substantial pre-load is required on the foil. That is, the individual foils must be loaded against the relatively movable bearing element opposed to the bearing element upon which the foils are mounted. It has been conventional to provide separate compliant stiffener elements or undersprings beneath the foils to supply this required preload as exemplified in U.S. Pat. Nos. 3,893,733, 4,153,315, and 5,116,143.
The undersprings typically include a number of corrugations. As shown in the '143 patent, these corrugations may be varied in width and stiffness to provide a support spring with a predetermined circumferential variation in support stiffness. Such a spring optimizes the spring force supporting the overlying foil so as to improve the match between the foil stiffness and the circumferential pressure distribution along the foil, and thereby maintain an optimum wedge shaped spacing between the shaft and the foil. By thus tailoring the foil support stiffness to the pressure distribution, the load carrying capacity of the bearing is increased.
Similarly, the undersprings may include means for reducing the exerted spring force at the axial edges of the foils to account for diminished dynamic fluid forces on the foil, and thereby prevent rubbing contact. One such means for reducing the spring force is described in U.S. Pat. No. 5,116,143 comprising cutouts or windows near the axial ends of the curvilinear support beams or corrugations. The cutouts are configured to approximate the spring force to the decrease in the overlying dynamic fluid pressure profile. The improved match of support stiffness to fluid pressure profile results in an increased overall load carrying capacity.
Variable stiffness undersprings can also provide load capacity benefits in a thrust bearing application. One such arrangement shown in U.S. Pat. No. 5,110,220 includes a foil thrust bearing underspring disk having a radially increasing spring force or load capacity. The underspring disk includes sets of three spring sections spaced apart radially, with the outer spring section having a greater spring force resilience than the radially inner spring sections. For example, the spring sections each include a plurality of corrugations, and the peak to peak length of the corrugations is shorter for the radially outer spring section than for the radially inner spring sections.
Although foil bearings of the type described above are able to accommodate some eccentricity of the rotating member and provide a degree of cushioning and dampening effect, large misalignment between the relatively rotatable members can alter the bearing load distribution. Misalignment can occur particularly in high-speed or high-temperature applications such as might be associated with a gas turbine engine, as a result of thermal gradients and pressure loads causing distortion of the housing and shafting. In addition, the more complex and the greater the number of components making up the structure, the greater the likelihood of misalignment at the bearings. The resulting altered dynamic pressure load direction on the foils can substantially reduce the load carrying capacity of the bearings, and ultimately result in break down of the film and damage to the bearing.
Overload of conventionally mounted foil journal bearings can also occur as a result of an instability at certain critical shaft speeds inherent in the shafting and structure design. Substantial radial shaft deflection at the bearing locations can occur when the shafting is operated at rigid body critical speeds, or multiples thereof. The radial deflection takes the form of a whirl of the shafting, where the whirl speed is less than the shaft rotation speed. The shaft whirl causes the pressure loading distribution on the bearings to depart from the optimal distribution designed for, resulting in a breakdown of the film and decrease in load carrying capacity.
Accordingly, a need exists for a foil journal bearing and a foil thrust bearing for use in high speed turbomachinery which can accommodate large relative misalignments of the housing and shafting without significant loss in load carrying capacity.
Need also exists for a foil journal bearing for use in high speed turbomachinery that can accommodate variations in a pressure load direction on the foils without loss of load carrying capacity.
Yet another need exists for a foil bearing that provides an optimized compliant backing for the underspring and foils without use of a variable stiffness underspring.